1. Field of the Invention
The present invention relates to an antenna device for receiving and/or transmitting dual polarized electromagnetic waves, comprising at least one antenna element located at a distance from a ground plane layer of electrically conducting material, and a feed network having two feed elements being adapted to transfer said electromagnetic waves from and/or to said antenna element.
2. Description of the Related Art
Such antennas are used i.a. for the transfer of microwave carriers in telecommunication systems, in particular in base stations for cellular mobile telephones. The capacity of the system is improved by the provision of two separate channels, obtained by orthogonal polarization, for each particular frequency or frequency band. However, in order to obtain the desired diversity, it is essential that the isolation between the two channels is very good.
In an antenna device disclosed in the pending Swedish patent application 9700401-4, an improved isolation between the two channels has been obtained by arranging, along a longitudinal row of antenna elements, in the vicinity of a gap between a respective pair of adjacent antenna elements, parasitic elements having a longitudinal extension substantially in parallel to the centre line of the row. In this way, the mutual electromagnetic coupling between the adjacent antenna elements, which would deteriorate the isolation has been reduced. Also, the isolation between the two channels within each one of the antenna elements is retained with such an arrangement.
However, a problem has arisen when trying to achieve a wider beam angle, such as 90.degree., by making the antenna device, and in particular the ground plane layer, more narrow. Then, it has turned out that the isolation between the two channels within each one of the antenna elements becomes insufficient, whereas the isolation between adjacent antenna elements can be retained even without parasitic elements provided that the distance between adjacent antenna elements is large enough.
Attempts have been made to vary the shape of the parasitic elements, but such attempts have failed to give any improvement.